int max(int a, int b)
{
    return a > b ? a : b;
}
int maxDepth(struct TreeNode* root) {
    if (root == NULL)
    {
        return 0;
    }
    int maxleft = maxDepth(root->left);
    int maxright = maxDepth(root->right);
    return max(maxleft, maxright) + 1;
}